At the 16th Conference of the Parties (COP) in 2010, developed countries formalised a collective climate finance commitment made previously in Copenhagen of “mobilising jointly USD 100 billion per year by 2020 to address the needs of developing countries...from a wide variety of sources, public and private, bilateral and multilateral, including alternative sources” (UNFCCC, 2010). However, there is currently no definition of which “climate” activities, flows, or other interventions could count towards the USD 100 billion; what “mobilising” means; or even which countries are covered by this commitment. The paper examines different definitions used by 24 key actors in climate finance to quantify the level of private climate finance mobilised by their interventions, as well as the methods used to track such private climate finance. Key findings are that i) methodologies to assess and estimate mobilisation vary widely, and ii) considerable risk of double-counting exists.